Many times in the history of producing wells, a lateral opening must be milled in the casing in order to continue production from an existing well. In the past it has been advantageous to be able to set a whipstock and mill a window without removing the production tubing. These techniques involve the use of a retrievable whipstock which is insertable through tubing. A good example of a through tubing retrievable whipstock is U.S. Pat. No. 5,909,770. In some instances in the past, a through tubing non-retrievable whipstock has been used in a multiple trip system for milling a window in a casing. In U.S. Patent Re 36,526 a through tubing non-retrievable whipstock is delivered through tubing and anchored in the casing. A separate trip is involved in delivering the mill or mills to mill the window in the casing.
In the past, whipstocks have been oriented downhole using measurement while drilling technology known as MWD. MWD tools required high flow rates for operation in orienting the whipstock appropriately. In the past, mills have been driven by downhole motors, generally of the progressing cavity type, involving a fixed stator and a rotating rotor driven by fluid flow through the stator.
One of the impediments in the past to running one-trip through tubing systems for milling windows, has been that use of applied pressure to set a whipstock anchor if delivered through the downhole motor would start the motor turning, which would prematurely break the mill loose from the whipstock prior to proper setting of the whipstock or it would alternatively rotate the whipstock. Accordingly, in developing the one-trip through tubing window milling system of the present invention, a motor lock has been developed for the downhole motor to prevent movement of the rotor as the anchor for the whipstock is being set. The apparatus and method of the present invention also envision hydraulically setting an anchor for the through tubing whipstock while having a way to retrieve the whipstock after the window is milled. The hydraulic anchoring assembly is preferred, particularly in deviated well applications due to the difficulties in properly actuating mechanically any anchor for the whipstock. The retrieval of the whipstock after the window milling necessarily involves release of the whipstock anchor to facilitate the removal of the whipstock through tubing. Accordingly, the present invention truly discloses a one-trip through tubing system for window milling whose details will be apparent to those of ordinary skill in the art from reading the detailed description of the preferred embodiment which appears below.